This invention relates to electronic phase shifters, and more particularly, to voltage-tunable dielectric microstrip phase shifters.
Prior to 1950, most phase shifters were mechanical. Electronic phase shifters became more important thereafter with the need for a steerable antenna beam (phased array antenna technology), especially for military applications. Lately, this has also become important in commercial telecommunications, i.e. satellite communications, and smart antenna technology for mobile telephony. Electronic phase shifters come in two varieties: continuously adjustable phase shifters and discrete stepped phase shifters. The latter usually employ pin diodes or low power transistors such as MESFETs as electronic switches. The former can be constructed using various technologies, including: (1) the use of tunable dielectric materials such as ferrites or ferroelectrics, etc.; (2) GaAs active phase shifters; (3) magnetostatic wave time delay phase shifters; and (4) MMIC phase shifters employing MESFETs and varactors.
Tunable phase shifters using ferroelectric materials are disclosed in U.S. Pat. Nos. 5,307,033, 5,032,805, and 5,561,407. These phase shifters include a ferroelectric substrate as the phase modulating element. The permittivity of the ferroelectric substrate can be changed by varying the strength of an electric field applied to the substrate. Tuning of the permittivity of the substrate results in phase shifting when an RF signal passes through the phase shifter. The ferroelectric phase shifters disclosed in those patents exhibit high conductor losses, high modes, high DC bias voltages, and impedance matching problems at K and Ka bands.
One known type of phase shifter is the microstrip line phase shifter. Examples of microstrip line phase shifters utilizing tunable dielectric materials are shown in U.S. Pat. Nos. 5,212,463; 5,451,567 and 5,479,139. These patents disclose microstrip lines loaded with a voltage tunable ferroelectric material to change the velocity of propagation of a guided electromagnetic wave.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZnOxe2x80x9d. These patents are hereby incorporated by reference. Copending, commonly assigned U.S. patent applications Ser. No. 09/594,837, filed Jun. 15, 2000, and Ser. No. 09/768,690, filed Jan. 24, 2001, disclose additional tunable dielectric materials and are also incorporated by reference. The materials shown in these patents, especially BSTOxe2x80x94MgO composites, exhibit low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Adjustable phase shifters are used in many electronic applications, such as for beam steering in phased array antennas. A phased array refers to an antenna configuration composed of a large number of elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. Phase shifters play a key role in operation of phased array antennas. The electronic beam steering concept applies to antennas used with both transmitters and receivers. Phased array antennas are advantageous in comparison to their mechanical counterparts with respect to speed, accuracy, and reliability. The replacement of gimbals in mechanically scanned antennas with electronic phase shifters in electronically scanned antennas increases the survivability of antennas used in defense systems through more rapid and accurate target identification. Complex tracking exercises can also be performed rapidly and accurately with a phased array antenna system.
U.S. Pat. No. 5,617,103 discloses a ferroelectric phase shifting antenna array that utilizes ferroelectric phase shifting components. The antennas disclosed in that patent utilize a structure in which a ferroelectric phase shifter is integrated on a single substrate with plural patch antennas. Additional examples of phased array antennas that employ electronic phase shifters can be found in U.S. Pat. Nos. 5,079,557; 5,218,358; 5,557,286; 5,589,845; 5,617,103; 5,917,455; and 5,940,030.
U.S. Pat. Nos. 5,472,935 and 6,078,827 disclose coplanar waveguides in which conductors of high temperature superconducting material are mounted on a tunable dielectric material. The use of such devices requires cooling to a relatively low temperature. In addition, U.S. Pat. Nos. 5,472,935 and 6,078,827 teach the use of tunable films of SrTiO3, or (Ba, Sr)TiO3 with high a ratio of Sr. ST and BST have high dielectric constants, which results in low characteristic impedance. This makes it necessary to transform the low impedance phase shifters to the commonly used 50 ohm impedance.
Low cost phase shifters that can operate at room temperature could significantly improve performance and reduce the cost of phased array antennas. This could play an important role in helping to transform this advanced technology from recent military dominated applications to commercial applications.
There is a need for electrically tunable phase shifters that can operate at room temperatures and at K and Ka band frequencies (18 GHz to 27 GHz and 27 GHz to 40 GHz, respectively), while maintaining high Q factors and having characteristic impedances that are compatible with existing circuits.
Phase shifters constructed in accordance with this invention include a substrate, a first electrode positioned on a surface of the substrate, a tunable dielectric layer positioned on a surface of the electrode, a microstrip positioned on a surface of the tunable dielectric layer opposite the substrate, an input for coupling a radio frequency signal to the microstrip, an output for receiving the radio frequency signal from the microstrip, and a connection for applying a control voltage to the electrode. In an alternative embodiment, a second electrode can be positioned on the surface of the substrate and separated from the first electrode to form a gap positioned under the microstrip.
Phase shifters constructed in accordance with this invention operate at room temperature. The phase shifters of the present invention can be used in phased array antennas at wide frequency ranges. The devices utilize low loss tunable dielectric materials.